This invention relates to structures for distributing toner in magnetic printing machines. More specifically, this invention relates to magnetic brush structures for transferring dry particulate toner to latent images on magnetic recording media.
Magnetic printing techniques are well known to the copier and facsimile arts. In a typical magnetic printing machine, electrical signals are applied to magnetic recording heads which induce magnetic field variations in the surface of a moving, magnetic recording medium. The field variations produce a latent magnetic image on the surface of the recording medium which is adapted for attracting and retaining magnetic ink particles. An ink toner, which may be in dry particulate form, is applied to the latent magnetic image and may be transferred to paper or other hard copy media. In many respects, magnetic printing is similar to the more common electrostatic, or xerographic, printing wherein toner particles are attracted to the electric fields created by latent charge image on a dielectric medium.
High quality magnetic printing requires that the toner particles be uniformly distributed on the surface of the recording medium. The toner consists of highly mobile, dust-like particles and care must be taken to prevent the spread of these particles to other components of the printing system with resultant degradation of the printed image.
Magnetic brush structures have, for many years, been utilized for the transfer of toner in electrostatic printing machines. The toner, comprising ferromagnetic materials and insulating resins, is attracted to the surface of a hollow, applicator cylinder rotatably disposed around a magnetic core. The magnetic core structure rotates with respect to the surrounding cylinder and carries the magnetic particles to the image surface in its magnetic field. More recently, magnetic brush structures having a fixed magnetic stator and a rotating applicator cylinder have been utilized. U.S. Pat. No. 3,553,464 to Abe describes a typical magnetic brush having a rotating core while U.S. Pat. No. 3,643,629 to Kangas describes magnetic brush systems having rotating applicator cylinders.
The magnetic brush structures of the prior art, while suitable for the application of toner to electrostatic latent images, produce strong magnetic field components in the plane of the recording medium surface. These field components will distort and erase the magnetic latent image in a magnetic printing machine and prevent the use of prior art magnetic brush structures in such magnetic printing machines.